When the transmission distance is long in an optical transmission system, an optical signal is attenuated. For this reason, one or more optical nodes are arranged in an optical transmission line so as to construct a long-distance optical transmission system. Each of the optical nodes is provided with an optical amplifier that amplifies an optical signal.
Moreover, the quality of an optical signal deteriorates after being transmitted through an optical fiber. In other words, the quality of an optical signal deteriorates under the influence of, for example, the chromatic dispersion and/or polarization mode dispersion of an optical fiber. Alternatively, the OSNR (Optical Signal-to-Noise Ratio) deteriorates at an optical receiver. For this reason, an optical node may be provided with an optical regenerator (optical regenerative repeater) that regenerates an optical signal.
FIG. 1A is a diagram illustrating an example of an optical transmission system. In the optical transmission system of FIG. 1A, an optical transmission device 1 and an optical transmission device 2 are connected by an optical transmission line 3. The optical transmission line 3 is an optical fiber cable. On the optical transmission line 3, optical nodes 4a, 4b, 4c, and 4d are arranged. Each of the optical nodes 4a, 4b, 4c, and 4d is provided with an optical amplifier to amplify an optical signal.
In the above-described optical transmission system, an optical transmitter of the optical transmission device 1 transmits an optical signal of specified bitrate. The optical nodes 4a, 4b, 4c, and 4d amplify and relay the optical signal, and the optical receiver of the optical transmission device 2 receives the optical signal that is transmitted through the optical transmission line 3.
Comparing with the optical transmission system of FIG. 1A, the optical transmission system of FIG. 1B has an extended optical transmission line 3 and an additional optical node 4e. In other words, the optical transmission system of FIG. 1B has an increased number of spans compared with the optical transmission system of FIG. 1A. Here, as the optical transmission line 3 is extended, the quality of an optical signal (OSNR, residual chromatic dispersion, polarization mode dispersion or the like) deteriorates at the optical transmission device 2. For this reason, this optical transmission system is provided with an optical regenerator 5 to regenerate an optical signal. In the example of FIG. 1B, the optical regenerator 5 is connected to the optical node 4d. The optical regenerator 5 has 3R (Regenerating, Retiming, Reshaping) functions. Accordingly, the quality of an optical signal improves at the optical transmission device 2.
FIG. 1C illustrates the state in which the characteristics of the optical transmission line 3 are deteriorated between the optical nodes 4c and 4d in the optical transmission system of FIG. 1A. Also in this case, the quality of an optical signal deteriorates at the optical receiver of the optical transmission device 2. For this reason, the optical transmission system is also provided with an optical regenerator 5 in order to secure a specified level of quality at the optical transmission device 2. In the example of FIG. 1C, the optical regenerator 5 is connected to the optical node 4c. 
For example, Japanese Laid-open Patent Publication No. 06-77962, Japanese Laid-open Patent Publication No. 10-75279, Japanese Laid-open Patent Publication No. 2004-356742, Japanese National Publication of International Patent Application No. 2008-533822, and Japanese Laid-open Patent Publication No. 2008-278488 describe the related art. Moreover, OFC/NFOEC 2009, NTuB5, Axel Klekamp, Olivier Rival, Annalisa Morea, Roman Dischler, Fred Buchali “Transparent WDM Network with Bitrate Tunable Optical OFDM Transponders” describes optical OFDM transponders in which the bitrate is turnable.
As described above, the transmission distance can be extended (i.e., the number of spans can be increased) by arranging an optical regenerator in the optical transmission line of the optical transmission system. Alternatively, the deterioration of an optical fiber can be repaired by providing an optical regenerator in the optical transmission system.
However, an optical regenerator converts an optical signal into an electric signal, and performs the 3R processes to that electric signal. Then the optical regenerator converts the electric signal to which the 3R processes are performed into an optical signal, and outputs the resultant optical signal to the optical transmission line. In other words, the optical regenerator is provided with a photo/electric conversion element and an electric/photo conversion element. For this reason, optical regenerators are expensive, and the circuit size tends to be large. In particular, in WDM networks in which a WDM optical signal is transmitted, it is preferable for an optical regenerator to have a photo/electric conversion element and an electric/photo conversion element for every wavelength. In such cases, an optical regenerator becomes very expensive.